Question

The $d$ electron configuration of $Cr ^{2+}, Mn ^{2+}, Fe ^{2+}$ and $Ni ^{2+}$ are $3 d ^{4}, 3 d ^{5}, 3 d ^{6}$ and $3 d ^{8}$ respectively. Which one of the following aqua complex will exhibit the minimum paramagnetic behaviour? (At. Nos. $Cr =24, Mn =25, Fe =26, Co =27$ )

• A. $[Fe(H_2O)_6]^{2+}$
• B. $\left[ Ni \left( H _{2} O \right)_{6}\right]^{2+}$
• C. $[Co(H_2O)_6]^{2+}$
• D. $[Mn(H_2O)_6]^{2+}$

Answer 1

Answer: (C)

$[Co(H_2O)_6]^{2+}$

Answer 2

As magnetic moment is $\sqrt{n(n+2)}$ where $n$ is no. of unpair electron.
So ion with minimum no. of unpair electron will have minimum paramagnetism.
$=\left[ Co \left( H _{2} O \right)_{6}\right]^{2+}$ have minimum paramagnetic behaviour.